To get this you need to play Datacore Demolition in any difficulty.At the part where you and the scientist get in the elevator wait until you reach the top Jack should be standing right there go unarmed by pressing up on the D-pad then press R-B to wepon swipe(make sure you have enough space) then it should say you have picked up Jack`s magnum. Trust me it's really cool you cannot keep it though.

Unlockable WeaponsThe amount of carrying capacity the weapon takes up is in brackets. The mission is where the weapon first appears in. To unlock a weapon start in the level the weapon is found at and obtain it and finish the level with it.